Why is it that so many English words, as one traces their etymologies, run through Icelandic as one goes back?
But English does have a good few words which share a common ancestor with Icelandic. Icelandic as the most conservative of the Scandinavian languages is relatively close to Old Norse, from which English borrowed while the vikings were in Britain. They include such seemingly native words as them, skirt, and sky.
Then, of course, English is actually related to Old Norse and Icelandic since they are all Germanic languages. This is where most of the similar words in English and Icelandic mentioned in other comments and answer really come from.
So broadly speaking there are three kinds of related words between English and Icelandic: Directly borrowed, via Old Norse, and descended from proto-Germanic.
I think it's what I know as red car syndrome, which urban dictionary calls blue car syndrome
Either you know Icelandic, or you recently noticed one Icelandic etymology in particular, and thereafter became particularly prone to notice others.
English was heavily influenced by the old norse language, and icelandic hasn't changed much from old norse over time so it's still very close to it.
Now I don't speak icelandic, but I speak swedish, which also comes from old norse, and even between swedish and english there are a lot of similarities.
Some examples of english-icelandic words that both come from old norse: Father - Faðir (fathir, roughly..) Mother - móðir (mothir, also roughly..) House - hús Knife - hnífur (both come from the old norse word hníf) Window - vindauga (both come from old norse and literally mean 'an eye towards the wind')
So.. yeah, hope that was helpfull.